A power transformer is an electrical device that transfers energy between two circuits through electromagnetic induction. The power transformer can step up, or step down, voltage by a numeric value (the transformer ratio). The transformer has a primary side and a secondary side. During ordinary operation, the power is the same on the primary side and the secondary side. Because power is voltage times current, during ordinary operation, the current entering the power transformer on the primary side is the same as the current leaving the power transformer on the secondary side, scaled by the transformer ratio. Differential protection of power transformers compares the current entering the transformer and the current leaving the power transformer. A difference between the current entering and leaving the power transformer may indicate the presence of a fault within the power transformer.